Abiity added to turn off the screen.

Added ability to turn off screen and still have the screen preview work!
If you
press volume down the screen will go black and all hardware keys stop
working. To get the app
to come back up and turn keys back on press volume up.

NOTE:  It seems for some reason I have to press the power button once
and then volume up otherwise
it does not recoongize the voulume up key press.   I may have to look
into this further latter on.